1. Field of the Invention
This invention relates to an image forming system and in particular to an electrophotographic image forming system using a dual-component inversion developing system, such as a copier or a laser printer.
2. Description of the Related Art
With the electrophotographic image forming system such as a copier or a laser printer, it is not easy to always hold the image concentration constant because of characteristic variations of photoreceptors and developers and characteristic change of photoreceptors and developers accompanying change in the use environment of temperature, humidity, etc. For a full-color image formed by superimposing toners of multiple colors on each other, particularly it is difficult to stabilize the image concentration because the toner characteristics for each color affect the image concentration. For example, the photo sensitivity of a photoreceptor is degraded with time, thus if the toner concentration in a dual-component developer is optimum, the image concentration tends to lower with long-term use. Therefore, the toner amount is increased for recovering the image concentration; on the other hand, if the toner concentration exceeds the optimum value, image quality failures of fog, character crush, etc., are caused.
In view of the electrophotographic characteristics, hitherto, various improvement measures for stabilizing the image concentration have been proposed. For example, in image forming systems described in Japanese Patent Unexamined Publication Nos. Sho. 61-254961 and Hei. 3-98064, the surface potential of a photoreceptor is detected by a potential sensor (ESV) and charge and exposure conditions are controlled so as to converge the surface potential on a target value. The concentration of a concentration detection pattern (toner patch image) formed after the surface potential is thus made constant is detected by a concentration detection sensor and the toner amount to be supplied to a developing device is controlled based on the concentration detection result.
If the toner patch image is thus formed after the surface potential of the photoreceptor is set to the target value, the detected concentration information of the toner patch image is not affected by fluctuation of the photo sensitivity of the photoreceptor. That is, light and dark of the toner patch image provided from the concentration information accurately reflect the toner amount in the developer (toner concentration) and control of the toner supply amount is facilitated; resultantly, a desired image concentration can be reliably provided according to the charge and exposure conditions.
On the other hand, a system for controlling so as to stabilize the toner concentration in a developer to a certain degree for preventing the image quality failures from occurring is described, for example, in Japanese Patent Unexamined Publication No. Hei. 4-152371. The system comprises a toner concentration sensor installed in a developing device to hold the toner concentration constant. Controlling the toner amount to be supplied to the developer so as to make the toner concentration constant based on the detection result of the toner concentration sensor is a general technique adopted for stabilizing the toner concentration.
Hitherto, consumed toner has been sensed by a consumed toner sensor installed in a toner supply device to manage the toner amount in the toner supply device. However, in recent years, the number of sensors has been reduced, for example, by recognizing consumed toner when a signal indicating that the toner concentration lowered several consecutive times is output by the toner concentration sensor. To reduce the number of sensors, a detection system for counting the number of image dots and assuming that images of a predetermined amount have been formed and that the toner has been consumed if the number of image dots exceeds a certain value is proposed in Japanese Patent Unexamined Publication No. Hei. 2-39178.
However, the conventional systems involve the following problems: First, the method of controlling the toner supply amount based on the toner concentration detected by the toner concentration sensor cannot precisely control the toner supply amount if the toner charge amount changes due to environmental fluctuation of temperature, humidity, etc., or degradation with time, because even if the toner concentration is optimum, when the toner charge amount changes due to degradation of carriers, the bonding strength of the toner and the carriers changes and the toner move amount onto the photoreceptor changes.
In the method of recognizing consumed toner when the toner concentration lowered several consecutive times, if processing of a large image amount of an image occurs, the concentration temporarily lowers, but in fact, sufficient toner may remain. Thus, the consecutive number of times the toner concentration has lowered or the concentration lowering level as the consumed toner determination criteria needs to be corrected, resulting in complicated control. Further, the method of counting the number of image dots and estimating the toner consumption amount contains a number of variables such that the relationship between the number of dots and the toner consumption amount depends on the image type; a practical problem remains unsolved.
According to the consideration results, it is desired that the system for holding the surface potential of the photoreceptor constant and then detecting the image concentration comprises the toner concentration sensor for detecting consumed toner. However, since the potential sensor and the toner concentration sensor are expensive, it is not adequate to apply a large number of sensors to a so-called low-speed machine; another problem that low-speed machine performance cannot be improved is caused.
On the other hand, as described above, the toner concentration of a dual-component developer, namely, the percentage of toner weight to total weight of toner and carriers, is extremely important on stabilizing the image quality. Although the toner of the developer is consumed, the carriers are not consumed, thus the toner concentration changes. Thus, an image forming system using a dual-component developer is provided with a developer concentration controller (ATR) for precisely predicting the toner consumption amount of the developer and replenishing toner in response to the predication result for always controlling the toner concentration constant.
FIG. 39 is a diagram to show the general configuration of a conventional digital printer having a developer concentration control function. An image data preparation device 101 supplies a pixel signal having an output level corresponding to the pixel concentration for each pixel to a pulse width modulation circuit 102. The pulse width modulation circuit 102 forms a laser drive pulse PR of width (duration) corresponding to the output level for each input pixel signal and outputs the laser drive pulse PR. That is, it forms a drive pulse of wider width in response to a high-concentration pixel signal, a drive pulse of narrower width in response to a low-concentration pixel signal, and a drive pulse of intermediate width in response to an intermediate-concentration pixel signal.
The laser drive pulse PR output from the pulse width modulation circuit 102 is supplied to a semiconductor laser 104 and causes the semiconductor laser 104 to emit light by the time corresponding to the pulse width. Therefore, the semiconductor laser 104 is driven for longer time for a high-concentration pixel and for shorter time for a low-concentration pixel. The laser light emitted from the semiconductor laser 104 is scanned in the horizontal scanning direction by a polygon mirror 114 and is applied through a reflection mirror 115 onto a photoreceptor drum 120 as an image carrier, forming an electrostatic latent image.
Electricity on the surface of the photoreceptor drum 120 is removed uniformly by an exposure device 118, then the surface is charged uniformly by a primary charger 117. Further, the surface of the photoreceptor drum 120 is irradiated with the laser light, forming an electrostatic latent image responsive to the image signal. The electrostatic latent image is developed to a visible image (toner image) by a developing device 111. The toner image is transferred by the action of a transfer charger 121 to a transfer material 123 held on a transfer belt 122 driven in the arrow direction by two rollers 113 and 124. The remaining toner left on the photoreceptor drum 120 is scraped out by a cleaner 119.
The output signal of the laser drive pulse PR output from the pulse width modulation circuit 102 is supplied to one input terminal of an AND gate 116 and a reference clock CLK is supplied to the other input terminal of the AND gate 116 from a reference clock source 125. Therefore, as many integration clock pulses Pa as the number corresponding to the pulse width Wn of the laser drive pulse PR, namely, as many clock pulses as the number corresponding to the concentration of each pixel are output from the AND gate 116, as shown in FIG. 40. A pulse accumulation device 103 integrates the number of integration clock pulses Pa for each pixel and sends the result to a CPU 106, which then converts the integration amount into a replenishment amount based on a data table entered in a RAM 105 and sends the replenishment amount to a motor drive circuit 107 as a toner replenishment signal. The motor drive circuit 107 drives a motor 108 by the time corresponding to the toner replenishment signal for turning a toner transport screw 110 in a toner replenishment tank 109 storing toner by the predetermined time for replenishing the developing device 111 with a proper amount of toner from the toner replenishment tank 109, thereby holding the concentration of the toner 112 in the developing device 111 constant.
In the conventional printer, the concentration of each pixel is converted into the pulse width of the laser drive pulse PR and the pulse width Wn is assumed to be the toner consumption amount to be counted by the reference clocks CLK. Thus, the concentration of each pixel and the toner amount predicted to be consumed to represent the pixel show a proportional relationship as indicated by dashed line A in FIG. 41.
In fact, however, toner is not attracted on an area where the latent image width is narrow (namely, the pulse width of the laser drive pulse PR is short), thus the toner consumption amount for the pixel concentration lessens. As the latent image width widens to some extent, toner is attracted reliably on the latent image, so that the pixel concentration and the toner consumption amount show a proportional relationship. Further, if the latent image width overwidens, the spacing between the latent image and the adjacent latent image becomes narrow and the latent images are made continuous, so that the toner consumption amount becomes constant regardless of the latent image width. Thus, the pixel concentration and the toner consumption amount actually show a nonlinear relationship like an S letter as indicated by solid line B in FIG. 41. Therefore, the conventional printer involves a problem that a large error occurs between the toner consumption amount predicted from the pixel concentration and the actual toner consumption amount.
Further, the laser drive pulse PR has a frequency of about 15 MHz. If an attempt is made to count the laser drive pulse PR with eight-time precision, 120-MHz reference clock is required. If peripheral circuitry is also made compatible with 120 MHz matching the reference clock, the circuit configuration becomes complicated and expensive.